all: give nested modules fully-qualified names

The two crypto modules are both named "asm". If both are included in a
single go.work (e.g., from `go work use -r .` in the repo), builds break
from "module asm appears multiple times in workspace".

Give these modules fully-qualified names to avoid conflicts. While we
are here, also expand the name of two other testdata modules. Those
modules don't currently conflict, but they have vague names at risk of
future conflicts.

Fixes golang/go#57769.

Change-Id: I2bd8a505051e92348d49560ec698ed921f2c81be
Reviewed-on: https://go-review.googlesource.com/c/go/+/461896
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

field/_asm/go.mod

@@ -1,4 +1,4 @@
-module asm
+module std/crypto/internal/edwards25519/field/_asm
 
 go 1.18